The present invention relates to methods of treatment of cellulosic fibers and particularly to methods for producing low fines content pulp for use in tissue, fluff and towel products.
Known methods of mechanical defibration of wood chips, impregnated or not, such as disc or grindstone defibration, produce substantial quantities of fines. High levels of fines, however, are detrimental to many products, such as tissue, fluff and towel products, because they cause dusting, drainage problems on the paper making machinery, poor fiber retention and decreased softness in the resulting product. Short fibers in the pulp for use in these products are also not desired.
Cellulosic fibers are sometimes treated in an apparatus comprising a pair of intermeshing screws in a treatment housing in communication at one end with an inlet for receiving the cellulosic fibers and at its opposite end with an outlet for the treated fibers. The screws counter-rotate in the area where the flights of the screws intermesh and that area constitutes a treatment zone. Treatment is conducted under compression and shear. This generally provides pulps having certain beneficial advantages. Such apparatus is commercially available under the trademark Frotapulper.RTM..
Methods of treating pulp using that apparatus have been effective in modifying pulp fibers by the application of compressive and shear forces on the pulp at high consistencies and high temperature. For example, with respect to kraft and sulfite pulps, this treatment results in a reduction in tensile strength, water retention value and swollen volume. Porosity, stretch and tear strength are increased. Rejects from chemical pulps may also be defibered using the foregoing described apparatus. Additionally, such apparatus has been used to disperse waste paper contaminants and modify the physical properties of such waste paper. In that connection, the apparatus defibers waste without producing substantial fines. Other uses of the foregoing apparatus have included removal of resin from pulps, the mixing of chemicals in pulp, defibration of flax, and defibration and development of properties of semi-chemical and chemimechanical pulps.
According to the present invention, there is provided a method for producing low fines content pulp for use in producing tissue, fluff and towel products. For purposes of this application, low fines content pulp may be defined as a pulp containing about 10% fines or less. To accomplish the foregoing, cellulosic fibrous chips are conventionally washed and squeezed to reduce their moisture content. The fiber material is then sequentially subjected to compressive and shearing forces while being impregnated with chemicals. For example, to form high freeness fluff, the washed and squeezed cellulosic fibers are refined by subjecting them to compressive and shearing forces at the same time a chemical, for example a mixture of sodium sulfite and DTPA, is added to the fiber. This compressive and shearing action may be characterized by low frequency high amplitude compression-relaxation cycles affording high energy transfer into the pulp at a low rate. After a certain residence time in a reaction chamber, the fibers are once again subjected to compressive and shearing forces of the same character and additional chemical, for example caustic, is added whereby the fibers are further softened. The fibers are subsequently subjected to additional compressive and shearing forces of like character in further similar stages of treatment while chemicals, such as bleaching agents, may be added until this controlled defibration produces a high freeness fluff with very few fines. Also, the treatment with the bleaching agent brightens the fibers. Thus, by sequentially subjecting the fibers in different treatment stages to compressive and shearing forces of this type while adding the appropriate chemicals, the fibers are gradually broken down without producing substantial fines. Rejects from the sequential treatment stages can be recycled back to previous treatment stages for further defibration and treatment.
Accordingly, it is a primary object of the present invention to provide a novel and improved method for producing low fines content pulp for use in tissue, fluff and towel products.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.